DAMASCUS -- International terrorism is out of vogue here in the Syrian capital, somewhat in the way long hemlines are disappearing in Paris. Western diplomats who muttered angrily a year ago about Syrian involvement in terror give Syria a clean bill of health today.

But the respite that seems to have been granted to the international travelers who are the terrorists' target of choice will not last long. As in the fashion world, the only constant in the terror business is change.

Terrorism, international opinion and diplomacy interact in what is becoming a clear cyclical pattern. Each phenomenon has its own force, and its own limits. The situation in Damascus is a case in point.

Responding to diplomatic sanctions, President Hafez Assad has finally shut down the Abu Nidal gang's operation here and has put under wraps the Syrian officials identified as having run the attempt to blow up an El Al airliner leaving London last year.

These officials have not been punished, but they are in limbo. The head of Air Force intelligence, Gen. Mohammed Kholi, the most senior figure mentioned in the El Al plot, is due to be banished to Moscow as ambassador, though he is resisting.

In return, the United States has sent back its ambassador after a 10-month hiatus. Except for Britain, Europe is also dropping the symbolic barriers that it erected around Syria in the wake of the El Al affair.

It is hard to argue with the Syrian assessment that Assad has paid a minimal price for improved relations with the West through this tightening up. His actions restore the tight grip that he likes to exercise in any event, and serve his interests in other ways.

Not everyone in official Washington is satisfied that Assad went far enough to justify a return to business as usual. There are those who argue that Syria should now be pushed to root out Abu Nidal's training camps operating in Lebanese territory under nominal Syrian control.

There is also the studied failure of the United States to emphasize the evidence gathered by Pakistan that Damascus airport was used as a staging point for the suspected Abu Nidal gunmen who attacked a Pan Am airliner in Karachi in September 1986. Private Syrian assurances that there was no government involvement in this have been quietly accepted by the United States.

This is distasteful. But those who argued in Washington for a new attempt at accommodation with Assad are undoubtedly right on the main count: What could be accomplished by international pressure on a regime like this one had been accomplished, and it was time to gather up the winnings, as meager as they may seem.

One of the pillars of Assad's survival for 17 years as the absolute ruler of a country known for the brutal volatility of its politics has been his image of solidity and immobility under pressure.

He avoids making choices until he must, and refuses to make them under the appearance of pressure. He acted against Abu Nidal and Gen. Kholi only after it became apparent to the Syrian political elite that he had his own reasons to do so.

But anyone who would hope that a permanent change in the cycles of terrorism is under way must consider the other pillar of Assad's rule. He is ready to respond with brute force and to crush opponents if they challenge him directly.

This is not unique to Assad. Politics will continue to be violent in an Arab world beset by domestic rivalries, inadequate political institutions and the conflicts with Israel and Iran. Like other Arab leaders who intend to survive in this environment, Assad will maintain the instruments he needs to eliminate his enemies -- no matter what international pressures are applied.

Those instruments can all too easily be turned, with or without the ruler's knowledge, against soft international targets when the opportunities, and reasons, present themselves.

The poison is in the system, just as the vulnerability is in our system. Sending the American ambassador back to Damascus will not be enough to change that.

Consider the view of George Habash, the founder of the Popular Front for the Liberation of Palestine, which pioneered Palestinian hijacking and hostage-taking before deciding it was too costly to the Palestinian cause in world opinion.

International operations "are becoming more difficult" because of improved security, Habash said this week. "But I cannot be optimistic. People are thinking about how to overcome these obstacles right now. The West will find itself in 1988 with a new rise in such actions."